He was guest soloist in Munich, Frankfurt, Milan (La Scala, in Arabella, 1970; Das Rheingold, 1973; Fidelio, 1978; and Lohengrin, 1981), and Lisbon.

He began his now legendary association with Wagner and his Heldentenor roles at the Bayreuth Festival in 1969, where he sang the Helmsman in Der fliegende Holländer.

The major Wagnerian roles followed in quick succession: Erik in 1970, Lohengrin in 1971, Walter in 1973, Parsifal in 1975, Siegfried in 1976, Tristan and Tannhäuser in 1981.

He also has sung non-Wagnerian roles, including Hermann in Tchaikovsky's The Queen of Spades, Florestan in Fidelio, Britten's Peter Grimes, and Verdi's Otello.

Kollo has not confined himself to grand opera, but made many television appearances, singing lighter repertoire, including an operetta he composed himself.